This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-288763, filed Sep. 21, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a piezoelectric thin film resonator and also to a frequency variable resonator where this piezoelectric thin film resonator is employed.
2. Description of the Related Art
In recent years, the market for mobile communication devices including portable telephone is increasing, and at the same time, the services available through mobile communications are increasingly made versatile. Due to this versatility, there is a trend that the frequency band to be utilized by mobile communication devices is gradually shifted toward high frequencies of the order of gigahertz (GHz) or more.
As for the filter which is useful at such high-frequencies, a surface acoustic wave (SAW) device, or a dielectric resonator where a bulk dielectric substance is employed as a resonator has been generally utilized up to date.
However, due to the limitation in the fine working of comb-shaped electrodes, and in consideration of the reliability of this fine working, it is now considered that the shifting of the frequency band of the SAW device toward higher frequencies still more is nearly no longer possible.
On the other hand, the dielectric resonator where a bulk dielectric substance is employed as a resonator is defective in that the dielectric resonator is generally not suited to miniaturization, so that it is not suitable for utilization in a mobile communication device which is inherently required to be light and small.
With a view to overcoming these problems, a film bulk acoustic resonator (FBAR) where the acoustic vibration in the direction normal to the surface of a piezoelectric thin film is utilized has been recently proposed in U.S. Pat. Nos. 6,060,818, 5,587,620, and 5,821,833, some kinds of FBAR being already available on the market. It is possible, according to this FBAR, to filter a high-frequency band of the order of 2 GHz without necessitating the fine working which the aforementioned SAW device necessitates. Furthermore, this FBAR is advantageous in that it can be greatly miniaturized as compared with the dielectric resonator where a bulk dielectric substance is employed as a resonator.
As for specific examples of piezoelectric thin film that can be utilized in this FBAR and the like, it is known to employ a thin film formed of aluminum nitride (AlN) or zinc oxide (ZnO). However, although it is possible, according to the thin film formed of aluminum nitride or zinc oxide, to secure a relatively large magnitude of mechanical quality factor (Qm), the thin film formed of aluminum nitride or zinc oxide is defective in that the electromechanical coupling coefficient (kt) with respect to the acoustic vibration in the direction normal to the surface of the thin film is relatively small and the band that can be filtered is relatively narrow.
It may be noted that the mechanical quality factor (Qm) represents a magnitude of loss of elastic energy in resonance frequency, and can be represented by the following equation:
Qmxe2x88x921=(f2xe2x88x92f1)/f0 
Wherein f0 is a frequency which makes the admittance become maximum in the vicinity of the resonance frequency, and f2 and f1 denote respectively a frequency which makes the admittance become a half of the maximum admittance.
Under the circumstances, it is now having attempted to employ a thin film of ferroelectric substance exhibiting a large electromechanical coupling coefficient such as lead titanate (PbTiO3) or lead zirconate titanate (Pb(Zi, Ti)O3:PZT). Since it is possible, according to these ferroelectric thin films, to obtain a large electromechanical coupling coefficient, it is expected to enlarge the frequency band of the filter.
However, since these ferroelectric thin films generally have a low mechanical quality factor, these ferroelectric thin films are defective in that the loss is increased.
There is also known an FBAR where a thin film made of aluminum nitride or zinc oxide is employed and which makes it possible to control the resonance frequency to a desired frequency through the application of an electric field onto a piezoelectric thin film. This FBAR employing aluminum nitride or zinc oxide is accompanied however with the problem that the voltage to be applied thereto is as high as 30 volts.
As explained above, according to the prior art, no-one has succeeded in providing a piezoelectric thin film resonator which is sufficiently large not only in electromechanical coupling coefficient but also in mechanical quality factor.
Further, according to the conventional piezoelectric thin film resonator, it is required to apply a very high voltage to a piezoelectric substance in order to make the frequency thereof variable.
An object of the present invention is to provide a piezoelectric thin film resonator which is sufficiently large not only in electromechanical coupling coefficient but also in mechanical quality factor.
According to one aspect of the present invention, there is provided a piezoelectric thin film resonator which comprises:
a first electrode;
a second electrode; and
a piezoelectric film which is interposed between the first electrode and the second electrode, and formed of an epitaxial ferroelectric thin film containing barium titanate, a spontaneous polarization of the epitaxial ferroelectric thin film being uniaxially orientated in a direction normal to a film surface.
According to another aspect of the present invention, there is provided a piezoelectric thin film resonator which comprises:
a first electrode;
a second electrode; and
a piezoelectric film which is interposed between the first electrode and the second electrode, and formed of an epitaxial ferroelectric thin film containing barium titanate, wherein a spontaneous polarization of the epitaxial ferroelectric thin film is uniaxially orientated in (001) azimuth and the in-plane two-dimensional lattice strain xm of the epitaxial ferroelectric thin film is confined to meet an expression of: xm greater than xe2x88x920.025.
According to a further aspect of the present invention, there is provided a frequency variable resonator which is capable of resonating at a plurality of frequencies through changes of a bias electric field, the filter resonator comprising:
any one of the aforementioned piezoelectric thin film resonators; and
a variable voltage source for applying a bias electric field between the aforementioned first and second electrodes.
According to a further aspect of the present invention, there is provided a piezoelectric thin film resonator which comprises:
a first electrode;
a second electrode; and
a piezoelectric film which is interposed between the first electrode and the second electrode, and formed of an epitaxial ferroelectric thin film which has a perovskite-type crystal structure, a spontaneous polarization of the epitaxial ferroelectric thin film being uniaxially orientated in a direction normal to the film surface.
According to a further aspect of the present invention, there is provided a piezoelectric thin film resonator which comprises:
a first electrode;
a second electrode; and
a piezoelectric film which is interposed between the first electrode and the second electrode, and formed of an epitaxial ferroelectric thin film which has a perovskite-type crystal structure, wherein a spontaneous polarization of the epitaxial ferroelectric thin film is uniaxially orientated in (001) azimuth and the in-plane two-dimensional lattice strain xm of the epitaxial ferroelectric thin film is confined to meet an expression of: xm greater than xe2x88x920.025.